Electrical energy is generated in electric generators as an embodiment of an electrodynamic machine. For this purpose, a rotor mounted rotatably about an axis of rotation is arranged within a stator, wherein the rotor generates a magnetic field which is moved around the axis of rotation. This rotating magnetic field induces an electric voltage in the winding arranged in the stator. Since generators experience a temperature increase during operation, they need to have suitable cooling. It is known to cool generators using air, oxygen or hydrogen. For this purpose, it is necessary to form the stator in the housing wall in such a way that the emergence of cooling medium is effectively avoided.
However, this means that the electrical energy generated in the stator needs to be passed to the outside out of the housing by means of so-called end-lead bushings, wherein the end-lead bushings should not permit any leakage with respect to the housing. The complexity involved in the production of the end-lead bushings and the arrangement thereof in the housing is comparatively great since, owing to the prevailing high voltages and currents, owing to the required short-circuit strength and owing to the requirements in respect of gas pressure and gas density, stringent electrical, thermal and mechanical requirements are set.
In general, six end-lead bushings are required for an electric generator. In such generators, there are three windings, which are also referred to as a three-phase winding and represent one phase of the three-phase AC supply. Each phase comprises a start point and an end point, wherein in each case the start point and the end point is passed out of the housing via in each case one end-lead bushing. In the case of three phases, six end-lead bushings are therefore required. For this purpose, a distinction is drawn between the start point of the winding, which is referred to as the phase terminal, and the end point of the winding, which is referred to as the neutral point terminal. It is known to connect the neutral point terminals which are passed out of the housing via the end-lead bushings to one another to form a neutral point outside the generator. The three phase terminals are coupled to a generator transformer for the transport of energy.
The insulation of the winding, in particular of the stator winding, needs to be checked regularly, which means that various electrical measurements such as, for example, resistance measurements and voltage tests, for example, need to be performed, wherein these measurements are performed offline. These measurements are taken individually for the phases. Therefore, the turn connections are passed out of the housing with the aid of so-called end-lead bushings.
The neutral point of a turbogenerator is short-circuited outside the housing and is covered by a neutral point box. The phase connections are in end-lead tubes with complex shielding and can only be made accessible with an extremely large amount of complexity in terms of dismantling. In order to be able to perform measurements on the individual phases of an electric generator, this end-lead bushing box needs to be opened and the neutral point, which includes screwed short-circuiting links, needs to be isolated and then connected again once the measurement has been performed.
The opening of the box, the isolation of the individual short-circuiting links which connect the end-lead bushings and form the neutral point, and the restoration of the contact points after the measurement operation are very complex.